The present invention relates to a gas turbine engine in which a proper clearance between the turbine blade tips and the seals or shrouds connected with the engine casing is maintained during various modes of engine operation.
It is well known in the prior art such as exemplified in U.S. Pat. Nos. 3,391,904, 3,583,824 and 4,019,320 to utilize cooling air from the compressor or fan in a gas turbine engine to control the expansion and contraction of blade tip seals and the supporting portion of the engine casing for optimum turbine performance. If the running clearances between the turbine blades and the seals are excessive, specific fuel consumption and power output from the engine suffer. On the other hand, adequate clearance must be maintained during all modes of engine operation to prevent destructive interference of the blades and seals.
The maintenance of a fixed seal clearance in the turbine section of an engine during all modes of operation is complicated by thermal transients in both the turbine rotor and the engine casing. As an engine starts up thermal growth of the casing generally lags far behind the relatively quick thermal growth of the rotor. To prevent interference between the blade tips and casing supported seals, a relatively large clearance is needed to accommodate the initial, rapid growth of the turbine rotor. When the thermal transients have leveled out, expansion of the engine casing has caught up with that of the rotor and again an excessive clearance will exist between the turbine blades and the seals. Such clearance in the steady-state condition as well as startup conditions allows hot combustion gases to leak past the turbine blades which reduces engine output and increases specific fuel consumption.
The seal clearance problem is further aggravated if the engine is accelerated from idle conditions soon after startup. The centrifugal growth of the rotor simply contributes to the rapid thermal growth rate. If cooling air is used in moderate amounts throughout the startup and high power operation to cool the engine casing, thermal growth rates of the casing are further restricted.
One solution employed in current engines utilizes large amounts of compressor air which under startup conditions is relatively warm and will initially aid expansion of the engine casing. This solution, however, is not altogether satisfactory due to the low temperature of the air employed.
Another solution also employs large amounts of compressor air which is ducted over the exterior of the engine casing during steady-state operation. The compressor air under these circumstances is relatively cool and shrinks the heated casing closer to the rotor. Both of these solutions are discussed in the above-referenced U.S. Pat. No. 3,583,824 but necessitate large amounts of air from the compressor. Thus while the turbine performance is enhanced compressor work is wasted.
It is also known from U.S. Pat. No. 3,736,751 to utilize the hot gases within the turbine section of the engine to control the positioning of a face sealing element. Hot gas from the engine escapes past the sealing element and flows through a thermally expandable control tube that supports the sealing element. Expansion of the tube closes the gap between the rotating blades and nonrotating seal element and reduces the flow of hot gases to the tube. Cold air is also fed through the tube and discharged to a low-pressure region by means of a restrictor. A preselected clearance or gap exists between the sealing element and blades when the flow of hot and cold fluid is balanced. In this prior art apparatus, however, the seal clearance involved is an axial clearance rather than blade tip clearance at the engine casing.
It is a principal object of the present invention to control clearance between the turbine blades and tip seals supported from the engine casing. An adequate but tight clearance is maintained throughout various modes of engine operation in spite of the different thermal growth rates associated with the turbine rotor and casing.